Method for the synthesis of ferrates

ABSTRACT

A method of synthesis of alkali metal ferrates and alkaline earth metal ferrates, in which a trivalent iron compound is mixed with potash and optionally persulphate, and the mixture is heated at a temperature in the range of about 250 to about 500° C. for about 1 to 10 hours. The invention also relates to the use of the ferrates obtained by this method.

[0001] The present invention relates to a method of manufacturing alkalimetal ferrates and alkaline earth metal ferrates, and to the use of theferrates thus obtained.

[0002] The invention can be applied notably in the field of watertreatment and in the field of alkaline batteries.

[0003] Ferrates are strong oxidising agents which are obtained byoxidation of iron in valence state III into iron in valence state VI.These products have been known for more than a century. They werereported for the first time in 1841 by Fremy, and were then described byStahl as from 1884 (J. Prakt. Chem., 32, 448, 1884).

[0004] Several methods of synthesis by chemical oxidation have beenproposed as from the 1950's. According to one of these methods, in J.Am. Chem. Soc., 73, 1379, 951, Fe (VI) salts are obtained by reaction ofa solution of soda and calcium hypochlorite with an Fe(III) salt such asFe(NO₃)₃. Another method, which is described in Anal. Chem., 26, 1957,1954, relates to obtaining an Fe(VI) salt such as BaFeO₄ byprecipitation of another Fe(VI) salt such as K₂FeO₄.

[0005] More recently, U.S. Pat. No. 5,284,642 describes a method for thesolid state synthesis of an alkali metal sulphatoferrate or alkalineearth metal sulphatoferrate, of formula M(Fe,S)O₄, in which M representstwo atoms of sodium or potassium, or a calcium or barium atom. Thismethod makes use of the reaction of ferrous sulphate, FeSO₄.7H₂O withcalcium hypochlorite and potash. An improvement of this method isdescribed in the application FR-A-2,800,056, wherein sodium hypochloriteis replaced by chlorine gas, Cl₂.

[0006] These methods do nevertheless have some drawbacks:

[0007] chlorine gas is a dangerous and noxious reagent,

[0008] chlorine-containing effluents, the treatment of which remainsproblematic, are produced in significant amounts,

[0009] yields of ferrate are low,

[0010] the chlorine-containing species can lead in alkaline media to theformation of chlorates, which are not very stable and are explosivecompounds,

[0011] the behaviour of the materials in the presence of chlorine and inthe hot is problematic. Only special and expensive alloys can be used.

[0012] A problem that the present invention proposes solving is toprovide a method of synthesis of alkali metal ferrates and alkalineearth metal ferrates which does not necessitate the use of reagentswhich are dangerous and/or harmful to health.

[0013] Another problem that the present invention proposes solving is toprovide a method of synthesis of alkali metal ferrates and alkalineearth metal ferrates which is economical due to the fact that it doesnot necessitate recycling or removing effluents which are delicate ordangerous to manipulate.

[0014] Another problem that the present invention proposes solving is toprovide a method which enables alkali metal ferrates and alkaline earthmetal ferrates to be obtained with a yield which is acceptable for anindustrial production.

[0015] It has now been discovered, in a surprising way, and this is thebasis of the invention, that the use of persulphate as oxidising agentin the synthesis of ferrates brings about a solution to the problems setforth above.

[0016] Thus, according to a first aspect, the invention relates to amethod of synthesis of alkali metal ferrates and alkaline earth metalferrates, which comprises the following steps:

[0017] a) a trivalent iron compound is mixed with potash and optionallya persulphate; and

[0018] b) the mixture is heated at a temperature in the range of about250 to about 500° C. for about 1 to 10 hours, preferably for about 4 to8 hours.

[0019] Advantageously, the mixture mentioned above is heated by applyinga temperature gradient. For example, it is possible to heat the mixturein a step b1) at a temperature in the range of about 250 to about 350°C. for about 1 to 5 hours. If the persulphate has not been added in stepa), it is added in a step b2), to the product obtained in step b1). Theaddition is generally done after cooling said product. It is alsopossible to add potash with the persulphate. In a last step b3), theproduct obtained in step b1) or in step b2) is heated at a temperaturein the range of about 375 to about 500° C. for about 1 to 5 hours.

[0020] The ferrate synthesised is then recovered in a conventionalmanner, and it is optionally converted into another ferrate (e.g. analkali metal ferrate can be converted into an alkaline earth metalferrate as indicated above).

[0021] The trivalent iron compound which can be used within the contextof the invention is a ferric salt for example, notably ferric sulphate(Fe₂(SO₄)₃), which is hydrated or not, or an iron oxide, notably in theform of a pigment, such as goethite (α-FeOOH) or haematite (α-Fe₂O₃).Ferric sulphate can be obtained according to the method described in theapplication WO 02/090270. Iron oxides can be obtained for exampleaccording to the methods described in the applications EP-A-1,064,226,EP-A-1,106,577, EP-A-1,132,343. The Fe(III) compound is advantageouslyin solid form. When this compound is in the form of a solution, thelatter is advantageously neutralised in order to precipitate and recoverthe iron compound in solid form. The neutralisation can be done forexample with the aid of soda, lime, or magnesia.

[0022] According to a particular embodiment of the invention, the iron(III) compound can be obtained from an iron (II) compound, notably aferrous salt. For example, a solution of ferrous sulphate (Fe(SO₄)₂),which is a by-product of the manufacture of titanium dioxide TiO₂, isneutralised as indicated above. The neutralised solution is thenoxidised, e.g. by means of hydrogen peroxide, air, oxygen-enriched air,or oxygen gas, and the iron (III) compound is recovered according toconventional techniques.

[0023] The persulphate is indifferently used in sodium form (Na₂S₂O₈),potassium form (K₂S₂O₈), or ammonium form ((NH₄)₂S₂O₈). It iscommercially available in sodium and potassium form. In general, theamount of persulphate is in a stoichiometric excess of about 1 to 15%with respect to the amount of iron (III) compound.

[0024] Potash (KOH) is generally commercially available as pellets ; itis used in a stoichiometric excess of about 0.5 to about 10% withrespect to the amount of iron (III) compound.

[0025] The method of the invention makes use of persulphate, which is astable compound which is not very toxic and has a good oxidising power.

[0026] Thus, according to a second aspect, the invention relates to theuse of sodium persulphate, potassium persulphate or ammonium persulphateas oxidising agent in the synthesis of alkali metal ferrates or alkalineearth metal ferrates.

[0027] The synthetic method in accordance with the invention enablesferrates of MFeO₄ type to be obtained, wherein M represents either twoalkali metal atoms, such as lithium, sodium, or potassium, or analkaline earth metal atom, such as magnesium, calcium, or barium. Thesynthetic method of the invention also enables ferrates of M(Fe,X)O₄type to be obtained, wherein M is as defined above and X is an atomselected from Al, Si, P, S, Cl, Mo, Mn, and Cr. However, with the viewto the use of the product in water treatment, the pollutant elementssuch as chromium are to be proscribed.

[0028] The ferrates obtained according to the method of the inventionare, due to their strong oxidising power, notably useful as watertreatment agents, notably agents for treating waste water (see thearticle in Rev. Sci. Eau 1996, 1:17-30, with regard to this), and ascathodes in alkaline batteries (see U.S. Pat. No. 6,387,569 with regardto this).

[0029] The invention is illustrated by the following Examples, which aregiven purely as an illustration.

EXAMPLE 1 Synthesis of Potassium Ferrate

[0030] 5 g of Fe(III), expressed in Fe₂O₃ equivalents, is weighed out,and is mixed with 3.5137 g of KOH. This mixture is placed in a nickelcrucible and is then heated in an oven at 300° C. for 4 hours. Aftercooling, 15.5 g of KOH and 28 g of K₂S₂O₈ is added, and the whole ishomogenised. The sample is put back in the oven at 400° C. for 3 hours.

[0031] After cooling in a desiccator, 47 g of product is obtained whichis then ground and analysed by X-ray diffraction. The diffractionprofile shows the presence of lines of K₂FeO₄ only.

EXAMPLE 2 Synthesis of Potassium Ferrate

[0032] 1 g of iron (III), expressed in Fe₂O₃ equivalents, is weighedout, which is mixed with 5.85 g of K₂S₂O₈ and 3.5 g of KOH in groundpellets. This mixture is placed in a nickel crucible and is then heatedin an oven at 300° C. for 3 hours, and then for 1 hour at 400° C.

[0033] The reaction can be represented in the following way:

10 KOH+Fe₂O₃+3 K₂S₂O₈→2 K₂FeO₄+6 K₂SO₄+5 H₂O

[0034] After cooling, 9.8 g of product is collected which is placed inthe desiccator and is then analysed by X-ray diffraction. Thediffraction profile shows the presence of KFeO₂ lines and K₂FeO₄ lines,and the formation of K₂SO₄.

EXAMPLE 3 Synthesis of an Iron (III) Compound

[0035] A solution containing FeSO₄ in acid medium is used as ironsource. The pH of the starting solution is 0.6 and the redox potentialequilibrates around E=−117 mV/ECS. This indeed confirms that the iron isin divalent form. This solution is neutralised by means of 30% soda atthe rate of 150 ml per 100 ml of iron (II) solution. Afterneutralisation, the pH attains the value of 5.8, while the redoxpotential of the solution equilibrates around −168 mV/ECS. This valueindicates that the iron is still in Fe(II) form.

[0036] The solution is then oxidised by means of hydrogen peroxide, atthe rate of 1.3 ml of 35% H₂O₂ per 50 ml of Fe(II) solution. The pHregulates during this oxidation to a value of 3.8. The end of theoxidation is indicated by the jump of potential which passes from −168mV to +511 mV/ECS. This value indicates that the iron is in Fe(III)form.

[0037] After one hour's agitation to ensure crystal growth, theresulting pulp is filtered. 6.5 g of yellow precipitate (goethite) isobtained which, when dried, can serve as source of iron in thepreparation of a ferrate according to the method described in Example 2.

[0038] The filtrate only contains 39.2 mg/l of iron which thus indicatesa high level of recovery of the iron.

1-11 (canceled).
 12. A method of synthesis of alkali metal ferrates andalkaline earth metal ferrates, comprising the steps of: a) mixing atrivalent iron compound with potash and a persulphate; and b) heatingthe mixture at a temperature in the range of about 250 to about 500° C.for about 1 to 10 hours.
 13. The method according to claim 12,comprising the steps of: b1) heating the mixture of step a) at atemperature in the range of about 250 to about 350° C. for about 1 to 5hours; and b2) heating the product of step b1) at a temperature in therange of about 375 to about 500° C. for about 1 to 5 hours.
 14. Themethod according to claim 12, in which the trivalent iron compound is aferric salt or an iron oxide.
 15. The method according to claim 14, inwhich the iron oxide is goethite or haematite.
 16. The method accordingto claim 12, in which the trivalent iron compound is obtained from adivalent iron compound.
 17. The method according to claim 16, in whichthe divalent iron compound is a ferrous salt.
 18. The method accordingto claim 12, in which the persulphate is sodium persulphate, potassiumpersulphate, or ammonium persulphate. ammonium persulphate.
 19. Themethod according to claim 12, in which a stoichiometric excess of potashof about 0.5 to 10% is used with respect to the amount of trivalentiron.
 20. The method according to claim 12, in which a stoichiometricexcess of persulphate of about 1 to 15% is used with respect to theamount of trivalent iron.
 21. A method of synthesis of alkali metalferrates and alkaline earth metal ferrates, comprising the steps of: a)mixing a trivalent iron compound with potash; b) heating the mixture ofstep a) at a temperature in the range of about 250 to about 350° C. forabout 1 to 5 hours; c) adding a persulphate to the heated mixture; andd) heating the product of step c) at a temperature in the range of about375 to about 500° C. for about 1 to 5 hours.
 22. The method according toclaim 21, in which the trivalent iron compound is a ferric salt or aniron oxide.
 23. The method according to claim 22, in which the ironoxide is goethite or haematite.
 24. The method according to claim 21, inwhich the trivalent iron compound is obtained from a divalent ironcompound.
 25. The method according to claim 24 in which the divalentiron compound is a ferrous salt.
 26. The method according to claim 21,in which the persulphate is sodium persulphate, potassium persulphate,or ammonium persulphate.
 27. The method according to claim 21, in whicha stoichiometric excess of potash of about 0.5 to 10% is used withrespect to the amount of trivalent iron.
 28. The method according toclaim 21, in which a stoichiometric excess of persulphate of about 1 to15% is used with respect to the amount of trivalent iron.